The refining and processing of crude petroleum into commercially useful petroleum products is a vital industry around the world. One of the most important petroleum products is the class of gasoline fuels. Gasoline fuels consist of a mixture of various hydrocarbon compounds. The concentration and chemical grouping of these hydrocarbon compounds determines the resulting fuel properties such as octane number and Reid vapor pressure (RVP).
Reid vapor pressure provides a volatility measurement of the gasoline. The octane number for a gasoline fuel is defined in terms of its knocking characteristics relative to a standard blend of isooctane (2,3,4-trimethylpentane) and n-heptane. Arbitrarily, an octane number of zero has been assigned to n-heptane and a rating of 100 to isooctane. Thus, an unknown fuel having a knocking tendency equal to a blend of 90% isooctane and 10% n-heptane, by volume, is assigned an octane number of 90.
During the manufacture of various grades of gasoline, it is useful to monitor the final product to be sure it possesses the desired physical properties, such as octane number. However, there currently is not a quick and inexpensive system for continuously monitoring a gasoline fuel composition. Instead periodic samples are normally taken from the process stream and analyzed. Occasionally, a gasoline fuel blend sold at one octane rating actually has a higher octane rating. This is uneconomical, since higher octane gasolines are more valuable to the refinery than lower octane gasolines.
It is also useful to control the concentration of various hydrocarbon ingredients in the gasoline blend. For instance, it is desirable to reduce the concentration of benzene, a known carcinogen, in gasoline. Yet, benzene concentration is often difficult to measure using conventional analysis. It is also desirable to minimize the olefin concentration. Olefins are unsaturated hydrocarbons (containing C.dbd.C bonds) which are photoreactive and contribute to smog formation. Olefins are also hard to measure accurately using conventional techniques. Finally, because xylene is a valuable gasoline ingredient, excess xylene should be minimized. It also would be valuable to identify and quantify the xylene isomers (para, meta, and ortho) present in the gasoline, but conventional analytical techniques cannot quickly distinguish between the xylene isomers. Thus, it would be a significant advancement in the art to permit continuous monitoring of the gasoline's chemical composition and physical properties.
Concentrated sulfuric acid is used by refineries in the alkylation process. The sulfuric acid concentration needs to be carefully controlled between 90% and 98% during the alkylation process. Currently, refineries manually take individual "grab-samples." Because of the unreacted hydrocarbon contamination in the acid, the samples require centrifuging to separate the acid. The acid is then titrated in the laboratory to determine exact acid content. These tests are typically run every four hours. Because of the time constraints with this type of testing, the refineries tend to run much higher acid concentrations than they would like to ensure that the process proceeds uninterrupted. Maintaining an excessively high acid concentration costs millions of dollars annually at each refinery.
When the acid concentration reaches 89%, it is removed from the process and hauled back to a chemical plant for recycling. This process of constantly removing, adding and transporting concentrated sulfuric acid is expensive and potentially dangerous. Because of the high cost of sulfuric acid, Amoco Oil Company calculates that for each 0.5% drop in average acid content in their alkylation process, they could save $5,000,000 per year throughout their entire production system. It is estimated that a continuous, on-line, analysis of acid concentration would enable the average acid concentration to be reduced by about 1%. Taking into account the additional savings from reduced transportation and handling costs, this total savings could average $12,000,000 per year for Amoco Oil. The sulfuric acid concentration problem is universal for the petroleum industry, and it has been estimated that a continuous, on-line chemical stream analysis could save the U.S. petroleum industry over $100,000,000 annually, with some of this savings returning to customers.
The alkylation process does not react the same with all alkene compounds; propylene compounds for instance, react much differently than do most other alkenes. The alkylation process needs a rapid, on-line control to properly react to changing feed stocks, to efficiently adjust acid levels, and economically produce petroleum products from a wide variety of incoming crude oil. Thus, there is a significant need in the art for a process and method which allows on-line analysis and control of acid content of such process streams.
Raman spectroscopy is an analytical technique which uses light scattering to identify and quantify molecules. When light of a single wavelength (monochromatic) interacts with a molecule, the light scattered by the molecule contains small amounts of light with wavelengths different from the incident light. The wavelengths present in the scattered light are characteristic of the structure of the molecule, and the intensity of this light is dependent on the concentration of these molecules. Thus, the identities and concentrations of various molecules in a substance can be determined by illuminating the substance with monochromatic light and then measuring the individual wavelengths and their intensities in the scattered light.
A continuing problem with Raman spectroscopy is the very low intensity of the scattered light compared to the incident light. Elaborate spectrometers, having high light gathering power and dispersion, high stray light rejection, and sensitive detectors, are required to isolate and measure the low intensity Raman scattered light. These instruments are costly and delicate, and are not well suited for use in industrial manufacturing or processing facilities. As a result, they have rarely been used outside of laboratory environments. Improvements in the fields of lasers, optical fibers, and filters enable one to remotely locate a fiber-optic probe from its laser light source and from its spectrometer.
It will be appreciated that there is a need in the art for an apparatus and method for analyzing industrial fluid streams, particularly those containing petroleum products, which provides quick and accurate results.